A multi-mode electrically variable transmission is an advantageous new transmission design that has the ability to reduce engine and electric motor losses at low as well as high vehicle speeds. However, depending on the implementation of the mode-changing mechanism, a multi-mode electrically variable transmission (“multi-mode EVT”) has potential disadvantages. For example, a multi-mode EVT may experience higher transmission spin losses due to clutch drag and multiple planetary gear set friction. Further, the gearing range within some multi-mode EVTs may limit the electric vehicle (“EV”) drive capabilities.
The gearing of a typical EVT must be designed with a compromise between city and highway driving in mind. As a result, the gearing of the typical EVT will often be higher than desirable for city driving in order to achieve adequate vehicle speeds during highway driving and lower than desirable for highway driving in order to provide adequate city driving performance. This compromise and the design of typical EVTs also means that exceedingly high torques are often applied to clutches within the EVT. Further, when operating purely under battery power without the propulsive force from the engine, the maximum final drive speed is limited by the component speeds of a planetary gear set within the EVT. Thus, in a battery electric vehicle or when used in a range-extended electric vehicle, EVTs typically have a limited top speed resulting from the city and highway gearing compromise made within the EVT. Therefore, an EVT is desirable that provides greater kinematic variability within the transmission while minimizing the compromise between city and highway performance.
In addition, a typical EVT is subject to increased mechanical and electrical losses during highway operation. These losses stem from internal friction within the EVT as well as the inherent electrical losses of using one electrical motor as a generator to power a second electrical motor. Thus, an EVT is desirable that provides for fixed gear operation with efficient highway cruising.
A typical EVT has limited reverse gear operation and relies solely upon one of its electric motors to provide reverse propulsion. This is problematic during situations in which electric power may be limited such as in extremely hot or cold climates. If electric power fails or provides inadequate propulsive force, the vehicle is simply unable to move in reverse. Thus, an EVT is needed that allows the EVT to harness the propulsive force of the engine for reverse gear operation, or that can use both electric motors of the EVT to provide reverse propulsion if battery power is adequate.
It is, therefore, desirable to provide an EVT that keeps the engine operating within its efficiency and/or power range while also providing satisfactory city and highway performance. It is also desirable to provide an EVT with reduced clutch torques and improved functionality in range-extended electric vehicles and battery electric vehicles. It is also desirable to provide an EVT with a fixed gear operating capability for improved highway cruising and a reverse gear for vehicle reverse operation.